5.4 Drilling and Drill Press

Drill can be defined as a rotary end cutting tool having one or more cutting lips, and having one or more helical or straight flutes for the passage of chips and the admission of a cutting fluid. There are several hole-making operations carried out on the drill press as follows:

Drilling. Drilling involves selecting the proper twist drill or cutter for the job, properly installing the drill into the machine spindle, setting the speed and feed, drilling a smaller pilot hole, and drilling the hole to specifications within the prescribed tolerance. Drilled holes are always slightly oversized, or slightly larger than the diameter of the drill’s original designation.

Reaming. Reaming can be performed on a drilling machine. It is difficult, if not impossible, to drill a hole to an exact standard diameter. When great accuracy is required, the holes are first drilled slightly undersized and then reamed to size. Reaming can be done on a drilling machine by using a hand reamer or using a machine reamer. When you must drill and ream a hole, it is best if the setup is not changed.

Tapping. Tapping is cutting a thread in a drilled hole. Tapping is accomplished on the drilling machine by selecting and drilling the tap drill size, then using the drilling machine chuck to hold and align the tap while it is turned by hand. The drilling machine is not a tapping machine, so it should not be used to power tap. To avoid breaking taps, ensure the tap aligns with the center axis of the hole, keep tap flutes clean to avoid jamming, and clean chips out of the bottom of the hole before attempting to tap.

Counterboring. Counterboring is the process of using a counterbore to enlarge the upper end of a hole to a predetermined depth and machine a recess at that depth. [6] Counterbored holes are primarily used to recess socket head cap screws and similar bolt heads slightly below the surface.

Countersinking. Countersinking is an operation in which a cone-shaped enlargement is cut at the top of a hole to form a recess below the surface. A conical cutting tool is used to produce this chamfer. When countersinking, the cutter must be properly aligned with the existing hole. Countersinking is useful in removing burrs from edges of holes, as well as accommodating the countersunk screw head.

Spot-facing. Spot-facing is basically the same as counterboring, using the same tool, speed, feed, and lubricant. Spot-facing is the smoothing off and squaring of a rough or curved surface around a hole to permit level seating of washers, nuts, or bolt heads .The operation of spot-facing is slightly different in that the spot-facing is usually done above a surface or on a curved surface. [7] When rough surfaces, castings, and curved surfaces are not at right angles to the cutting tool, great strain may occur on the pilot hole and counterbore, which can lead to broken tools. Both counterboring and spot-facing can be accomplished with standard counterbore cutters.

Boring. Boring is conducted when a straight and smooth hole is needed occasionally which is too large or odd sized for drills or reamers. A boring tool can be inserted into the drilling machine and bore any size hole into which the tool holder will fit. A boring bar with a tool bit installed is used for boring on the larger drilling machines. To bore accurately, the setup must be rigid, machine must be sturdy, and power feed must be used. Boring is not recommended for hand-feed drilling machines. Hand feed is not smooth enough for boring and can be dangerous, because the tool bit could catch the workpiece and throw it back to the operator.

There are many different types or configurations of drill presses, but most drill presses will fall into four broad categories: sensitive bench type, upright type, radial type, and special purpose type.

Rigid and accurate construction of drilling machines is important to obtain proper results with the various cutting tools used. The sensitive drilling machine construction features are discussed in this section because its features are common to most other drilling machines (Fig. 5.4).

Base is the main supporting member of the machine. It is heavy gray iron or ductile iron casting with slots to support and hold work that is too large for the table.

Column may be made of gray cast iron or ductile iron for larger machines, or steel tubing for smaller bench drill presses. It supports the table and the head of the drilling machine. The outer surface is machined to function as a precision way of aligning the spindle with the table.

Chuck is used to clamp and fix the drill through a cone-surface locking mechanism.

Table can be adjusted up or down the column to the proper height. It can also be swiveled around the column to the desired working position. Most worktables have slots and holes for mounting vises and other workholding accessories. Some tables are semi-universal, meaning that they can be swiveled about the horizontal axis.

Head houses the spindle, quill, pulleys, motor and feed mechanism. The V-belt from the motor drives a pulley in the front part of the head, which in turn drives the spindle. The spindle turns the drill. Speeds on a stepped V pulley drive are changed by changing the position of the V-belt. Speeds on a variable-speed drive mechanism are changed by a hand wheel on the head. The spindle must be revolving when this is done.

Quill assembly makes it possible to feed or withdraw the cutting tool from the work. The spindle rotates within the quill on bearings. The quill moves vertically by means of a rack and pinion. Located on the lower end of the spindle is either a Morse tapered hole or a threaded stub where the drill chuck is mounted. For drilling larger holes, the drill chuck is removed and Morse tapered cutting tools are mounted.